For example, in general, a power module having an electronic part, such as a semiconductor chip, mounted thereon has a power module substrate including a ceramic substrate that is made of, for example, AlN (aluminum nitride) or Al2O3 (alumina), Si3N4 (silicon nitride), or SiC (silicon carbide), a circuit layer, which is a metal member provided on the upper surface of the ceramic substrate, and a metal layer, which is a metal member provided on the lower surface of the ceramic substrate. A semiconductor chip, which is a heating element, is provided on the circuit layer on the power module substrate, and a cooling heat sink is provided on the lower surface of the metal layer (see Patent Document 1).
In the power module, heat generated from the semiconductor chip is transferred to cooling water in the heat sink through the metal layer.
In the power module, when the ceramic substrate is made of Si3N4 that has mechanical characteristics better than those of AlN, for example, a bending strength higher than that of AlN, it is possible to reduce the thickness of the ceramic substrate.    [Patent Document 1] JP-A-2002-9212